The invention in a first aspect is, however, preferably concerned with the detection of sodium in molten aluminium or aluminium alloys, and although it will be appreciated that the invention is not limited thereto, for convenience it will be described with specific reference to these metals.
Sodium is often added to aluminium or aluminium alloys as a structural modifier in order to improve the physical properties of the metal. It is generally necessary to determine the concentration of the sodium in the metal melt so that the desired concentration may be arrived at.
Various designs of electrochemical sensor have been proposed in the past. For example, UK Patent No. 1470558 discloses an apparatus for detecting an element in a substance, in which a reference material is a solid electrolyte comprising a β-alumina compound of the element, or a solid compound of the element, such as a tungstate, molybdate or vanadate, separated from the substance by the β-alumina compound.
UK Patent No. 1602564 discloses a modification of the apparatus disclosed in the above mentioned patent, in which a β-alumina compound of the element to be detected is fused into the end of a tube of refractory material to provide a sealed tubular probe.
European Patent No. EP 0 679 252 B1 discloses a sensor for the measurement of trace elements in molten metals or alloys which has a solid electrolyte formed from zirconia toughened strontium β-alumina. The sensor may, for example, act as a sensor for sulphur, in which case it may incorporate a reference material comprising a mixture of molybdenum metal and molybdenum sulphide powders, which provides a fixed sulphur partial pressure against which the activity of the sulphur in the molten metal is measured.